A great variety of aqueous drilling and completion fluid compositions have been developed and used heretofore. Problems which have been commonly encountered in the use of such drilling and completion fluids are that when high temperatures are encountered, the fluids become unstable, that is the viscosity and other properties are lost or reduced, and when the fluids come into contact with water-sensitive clay-containing formations or strata, the clay-containing materials slough off the walls of the well bore into the fluids. The sloughing of formation materials into the drilling and completion fluids can cause borehole instability and hole enlargement.
Heretofore, the sloughing of clay-containing formation materials comprising water-sensitive shales into a well bore, when contacted by aqueous drilling or completion fluids, has been reduced by including potassium chloride or potassium hydroxide in the drilling or completion fluids. While aqueous fluids containing potassium chloride or potassium hydroxide are effective in reducing the swelling of clays and the sloughing of clay-containing materials when in contact therewith, a number of other problems are inherent in the use of such solutions.
Drilling fluids containing potassium chloride in the quantity required to provide sufficient potassium ion concentration to prevent clay swelling and sloughing of formation materials frequently have demonstrated unstable rheological and filtration control properties. The high chloride ion content also causes the fluids to be more corrosive to tubular goods in the well bore and creates disposal problems. In addition, high chloride ion levels in drilling and completion fluids make electric logs and other analytical procedures carried out therein more difficult to interpret. That is, the chloride-containing fluids create a conductive medium which surrounds the logging apparatus in the well bore during use and hampers the investigation of the formations adjoining the conductive medium.
The use of potassium hydroxide in lieu of potassium chloride brings about more stable drilling and completion fluids, but the quantity of potassium hydroxide that can be used must be limited to prevent the fluids from becoming excessively caustic and corrosive. The presence of hydroxyl ions in the fluids offsets the effectiveness of potassium ions therein because hydroxyl ions promote hydration and dispersion of clays. Further, dispersants must normally be used in drilling or completion fluids containing potassium hydroxide which bring about the adverse result of dispersing cuttings in the fluids.
Other potassium compounds such as potassium carbonate and potassium bicarbonate have been utilized in well treatment fluids, but such compounds also cause the fluids to be unstable, etc. It would be desirable to develop a method by which the formation surrounding a well bore could be stabilized against detrimental formation degradation while creating an environment that facilitates investigation of the formation by electric well logging.